Methods of this type are known in the prior art, as indicated notably in the patents FR-2 683 776, FR-2 787 066, FR-2 719 260, EP-0 743 204, and U.S. Pat. No. 5,497,324.
The patent FR-2 683 776 proposes a method for obtaining a signal which represents the surface of the road in real time, this method being based on detection of the relative displacements measured by a sensor which is sensitive to the travel of the body with respect to at least one wheel (spring travel).
The signal representative of the road profile is obtained by filtering.
Conventional methods, such as the Fast Fourier Transform (FFT), are then applied for spectral analysis of the filtered signal, and calculation of the means and deviations, on the basis of a classification that takes into account appropriate characteristics which are notably related to the road profile and type.
This method, which is incapable of processing measurement noise or of achieving a temporal localization of the detected frequencies, presents intrinsic limitations of precision.
The patent FR-2 787 066 concerns an active suspension system which comprises active support groups which are arranged between the body and the wheels of the vehicle.
Each one of these groups consists of a passive spring and of a regulation device which allows the possibility of an excursion, and which is series connected to the corresponding passive spring.
These groups are controlled as a function of the accelerations imposed on the body, where a low-pass filtering procedure applied to the signals produced by the accelerometers and displacement sensors permits taking into account the high frequency and low frequency components in a differentiated matter.
A frequency-dependent damping is obtained by frequency modulation of the travel acceleration signals, where this modulation is the result of the combined effect of a low-pass filter, an amplifier, and a summer.
This technique has several specific limitations and defects.
First, it ignores the delay between the time that the acceleration sensor detects the movement of the body and the time that the excitation is produced which generates this movement of the body, so that the frequency which is estimated by modulation is marred by systematic error.
Moreover, this technique is inappropriate for the identification and temporal localization of the instantaneous frequency.
Finally, the frequencies of the filters are chosen a priori, on a theoretical basis, and thus are not capable of undergoing adaptation, and the effect of the shock absorber is not processed in certain frequency bands.
The patent FR-2 719 260 describes a method for controlling shock absorber regulation devices as a function of a signal which originates from an acceleration sensor.
To separate the oscillation mode due to the body from the oscillation mode due to the wheel, this signal is split by low-pass filtering and band-pass filtering, where the low-pass filtering has a cutoff frequency between 15 and 20 Hz to include the natural frequency of vibration of the wheel, and the band-pass filtering has a band width between 0.75 and 2.5 Hz, centered around a frequency of 1.5-2 Hz, to take into account the oscillation mode due to the body.
The same remarks can be formulated as for the preceding cases: this technique does not enable taking into account errors in the model and measurement noise.
The patent EP-0 743 204 describes a method for control of a vehicle chassis that uses processing of signals delivered by an ABS sensor to construct an observation system which allows control and command of the vehicle suspensions.
This method is based on the idea that the speed of wheel rotation is proportional to the travel speed of the suspensions (in expansion or compression), to exploit the hypothesis that the frequency components of the wheel speed and suspension travel signals are the same.
This method, which uses a low-pass filtering with a cutoff frequency of 1 Hz for displacement of the body and a low-pass filtering with a cutoff frequency of 10 Hz for displacement of the wheel, calls for the same remarks as were made about the above-mentioned methods.
U.S. Pat. No. 5,497,324 proposes a method which is based on the use of the travel speed of the displacements between the wheel and the body for observation of a semi-active suspension.
Observation of the frequency is achieved by measuring the time which separates two changes in the direction of the load on this suspension, where the period is taken to be the inverse of the frequency to the nearest 2π.
A sign change half-period equal to 500 msec is thus taken as the manifestation of the oscillation mode due to the body, a sign change half-period equal to 50 msec is taken as the manifestation of the oscillation mode due to the wheel, and a half-period of intermediate duration is taken as the manifestation of an oscillation mode related to [passenger] comfort.
However, to the extent that, in the general case, the excitation frequency at the level of the ground has already undergone variation at the time when the sensor detects a movement of the body or of the wheel, this method comprises a systematic error.
Moreover, this method in theory is applicable only to sinusoidal signals which are not phase-shifted, and to periodic signals whose frequency does not vary instantaneously.
Finally, this method, insofar as the noise of the sensors and the modeling errors are concerned, calls for the same remarks as those formulated above.
In this context, the purpose of the invention is to propose a method which allows the evaluation, in a more precise and more rigorous manner than in the prior art, of the instantaneous frequency of a mechanical excitation exerted on a motor vehicle wheel by the uneven surface of the road on which a vehicle is driven.
For this purpose, the method of the invention comprises:                a filtering method which uses at least the first measurement time series to produce at least a first derived principal time series that consists of successive estimates of the instantaneous height of the chassis, and a second derived principal time series that consists of successive estimates of the instantaneous height of the wheel,        a time lag extracting procedure, which uses at least the first derived principal series and the second derived principal series to produce a preparatory time series, which consists of successive estimated values of an instantaneous time lag between the instantaneous height of the chassis and the instantaneous height of the wheel, where each estimated value of the time lag is obtained by the optimization of at least one correlation function of the first derived principal time series and the second derived principal time series, and        a frequency estimating procedure that uses the preparatory time series to produce a final time series which consists of successive estimated values of the instantaneous frequency of a mechanical excitation, where each frequency value is obtained, in an observation time window defined by the corresponding estimated value of the instantaneous time lag, as the frequency at which this instantaneous time lag constitutes an instantaneous phase shift.        
While it is useful to integrate the measurement noise and modeling errors, the filtering method uses at least the first measurement time series to produce a first derived time series comprising, in addition to the first derived principal time series, a first derived auxiliary time series which consists of successive variance values that are respectively associated with the estimates of the first derived principal time series, and a second derived time series comprising, in addition to the second derived principal time series, a second derived auxiliary time series which consists of successive variance values that are respectively associated with the estimates of the second derived principal time series, and where each estimated value of the time lag is obtained by the optimization of at least one correlation function correlating the first derived time series and second derived time series.
For example, at least some of the measurement samples elaborated during the course of the acquisition procedure and pertaining to an observable parameter represent a relative displacement of the chassis with respect to the wheel.
However, at least some of the measurement samples elaborated during the acquisition procedure and pertaining to an observable parameter can also represent an instantaneous acceleration of the chassis, or of the wheel.
The acquisition method can comprise the elaboration of at least a second measurement time series, consisting of successive measurement samples which represent successive values taken on by a second observable parameter related to the instantaneous height of the chassis and/or of the wheel.
It is preferred for the method of the invention to comprise a prior modeling procedure which comprises, for a physical model associated with the system consisting of the wheel, the suspension devices and the chassis, the elaboration of a first inverse transfer function which takes the first observable parameter as input signal and the instantaneous height of the chassis as output signal, and the elaboration of a second inverse transfer function which takes the first observable parameter as input signal and the instantaneous height of the wheel as output signal.
The prior modeling procedure can also comprise, for the physical model which is associated with the system consisting of the wheel, the suspension devices and the chassis, the elaboration of a third inverse transfer function which takes the second observable parameter as input signal and the instantaneous height of the chassis as output signal, and the elaboration of a fourth inverse transfer function which takes the second observable parameter as input signal and the instantaneous height of the wheel as output signal.
In a variant, the prior modeling procedure can comprise, for the physical model associated with the system consisting of the wheel, the suspension devices and the chassis, the elaboration of a fifth inverse transfer function which takes the second observable parameter as input signal and the instantaneous height of the chassis or of the wheel as output signal, and the elaboration of a sixth inverse transfer function which takes the second observable parameter as input signal and the first observable parameter as output signal.
In all the cases, the filtering procedure is very advantageously implemented using at least one Kalman filter, and possibly at least two or more Kalman filters.
Each correlation function of the first derived time series and the second derived time series can consist of an intercorrelation function for the first and second derived time series.
However, each correlation function correlating the first and second derived time series can also be constructed using a model based on third-order cumulants.
Whenever possible, the procedure for estimating the frequency elaborates, for example, each value of the final time series as the solution, for the corresponding estimated value of the instantaneous phase shift, of at least one equation representing the physical model associated with the system consisting of the wheel, the suspension devices and the chassis.
In the other cases, the procedure for estimating the frequency can elaborate each value of the final time series by the application of a Wigner-Ville transform or a Wigner-Ville pseudo-transform.
The method of the invention has numerous applications, and in particular it can be used for the control of the suspension devices of the vehicle, for the determination of the instantaneous inclination of the chassis, for the determination of the uneven surface of the road, for monitoring the inflation of the wheel, and for the determination of certain characteristics of vehicles such as mass transfer.